Multilayered shell catalysts for catalytic gaseous phase oxidation of aromatic hydrocarbons

ABSTRACT

A coated catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons comprises, on an inert nonporous support, a catalytically active composition comprising a defined amount of vanadium oxide, titanium dioxide, a cesium compound, a phosphorus compound, and antimony oxide, wherein the catalytically active composition is applied in two or more layers and where relative to the inner layer or inner layers the outer layer has an antimony oxide content which is from 50 to 100% lower and wherein the amount of catalytically active composition of the inner layer or layers is from 10 to 90% by weight of the amount of catalytically active composition, and can be used for preparing carboxylic acids and/or anhydrides, in particular phthalic anhydride; also specified is a production process for such catalysts.

The present invention relates to a coated catalyst for the catalyticgas-phase oxidation of aromatic hydrocarbons, comprising, on an inertnonporous support, a catalytically active composition comprising, ineach case based on the total amount of catalytically active composition,from 1 to 40% by weight of vanadium oxide, calculated as V₂O₅, from 60to 99% by weight of titanium dioxide, calculated as TiO₂, up to 1% byweight of a cesium compound, calculated as Cs, up to 1% by weight of aphosphorus compound, calculated as P, and up to a total of 10% by weightof antimony oxide, calculated as Sb₂O₃. In addition, it relates to aproduction process for these catalysts and to a process using thesecatalysts for preparing carboxylic acids and/or anhydrides andespecially phthalic anhydride.

It is known that many carboxylic acids and/or carboxylic anhydrides areprepared industrially by the catalytic gas-phase oxidation of aromatichydrocarbons such as benzene, the xylenes, naphthalene, toluene ordurene in fixed-bed reactors, preferably multitube reactors. Theseprocesses are used to obtain, for example, benzoic acid, maleicanhydride, phthalic anhydride isophthalic acid, terephthalic acid orpyromellitic anhydride. The usual procedure is to pass a mixture of agas comprising molecular oxygen, for example air, and the startingmaterial to be oxidized through a plurality of tubes arranged in areactor, with a bed of at least one catalyst being present in each tube.To regulate the temperature, the tubes are surrounded by a heat transfermedium, for example a salt melt. Despite this thermostatting, it ispossible for hotspots in which the temperature is higher than in theremainder of the catalyst bed to occur. These hotspots give rise tosecondary reactions such as the total combustion of the startingmaterial or lead to formation of undesirable by-products which can beseparated from the reaction product only with difficulty, if at all, forexample the formation of phthalide or benzoic acid in the preparation ofphthalic anhydride (PA) from o-xylene. Furthermore, the formation of apronounced hotspot prevents a rapid running-up of the reactor to thereaction temperature of the reaction since the catalyst can beirreversibly damaged above a certain hotspot temperature, so that theloading can be increased only in small steps and has to be monitoredvery carefully.

To reduce this hotspot, it has become customary in industry to arrangecatalysts having different activities in zones in the catalyst bed, withthe less active catalyst generally being arranged in the fixed bed sothat the reaction gas mixture comes into contact with it first, i.e. itis located toward the gas inlet end of the bed, while the more activecatalyst is located toward the gas outlet end of the catalyst bed. Thecatalysts of differing activity in the catalyst bed can be exposed tothe reaction gas at the same temperature, but the two zones of catalystshaving differing activities can also be thermostatted to differentreaction temperatures for contact with the reaction gas (DE-A 40 13051).

Catalysts which have proven useful for these oxidation reactions arecoated catalysts in which the catalytically active composition isapplied in the form of a shell to a support material which is generallyinert under the reaction conditions, e.g. quartz (SiO₂), porcelain,magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al₂O₃),aluminum silicate, magnesium silicate (steatite), zirconium silicate orcerium silicate or a mixture of these support materials. Catalyticallyactive constituents of the catalytically active composition of thesecoated catalysts are generally titanium dioxide in the form of itsanatase modification plus vanadium pentoxide. In addition, thecatalytically active composition may further comprise small amounts ofmany other oxidic compounds which, as promoters, influence the activityand selectivity of the catalyst, for example by lowering or increasingits activity. Examples of such promoters are the alkali metal oxides, inparticular lithium, potassium, rubidium and cesium oxides, thallium(I)oxide, aluminum oxide, zirconium oxide, iron oxide, nickel oxide, cobaltoxide, manganese oxide, tin oxide, silver oxide, copper oxide, chromiumoxide, molybdenum oxide, tungsten oxide, iridium oxide, tantalum oxide,niobium oxide, arsenic oxide, antimony oxide, cerium oxide andphosphorus pentoxide. Promoters which reduce the activity and increasethe selectivity are, for example, the alkali metal oxides, while oxidicphosphorus compounds, in particular phosphorus pentoxide, increase theactivity of the catalyst but reduce its selectivity.

According to the processes of DE-A 16 42 938 and DE-A 17 69 998, suchcoated catalysts are produced by spraying an aqueous and/or organicsolvent-containing solution or suspension of the constituents of theactive composition and/or their precursor compounds, which ishereinafter referred to as a “slurry”, onto the support material in aheated coating drum at elevated temperature until the amount of activecomposition as a proportion of the total weight of the catalyst hasreached the desired value. According to DE 21 06 796, the coatingprocedure can also be carried out in fluidized-bed coaters as aredescribed, for example, in DE 1280756. However, spraying in a coatingdrum and coating in a fluidized bed result in high losses sinceconsiderable amounts of the slurry are converted into a mist or parts ofthe active composition which has already been applied are rubbed offagain by abrasion and are carried out by the waste gas. Since theproportion of active composition in the total catalyst should generallyhave only a small deviation from the prescribed value because the amountof active composition applied and the thickness of the shell stronglyinfluence the activity and selectivity of the catalyst, the productionmethods indicated require the catalyst to be cooled, taken from thecoating drum or the fluidized bed and weighed at frequent intervals todetermine the amount of active composition applied. If too much activecomposition is deposited on the catalyst support, it is generally notpossible to carry out a subsequent, careful removal of the excess activecomposition without adversely affecting the strength of the shell, inparticular without crack formation in the catalyst shell.

To reduce these problems, it has become customary in industry to addorganic binders, preferably copolymers, advantageously in the form of anaqueous dispersion, of vinyl acetate/vinyl laurate, vinylacetate/acrylate, styrene/acrylate, vinyl acetate/maleate and vinylacetate/ethylene, to the slurry. The amounts of binder used are 10-20%by weight, based on the solids content of the slurry (EP-A 744 214). Ifthe slurry is applied to the support without using organic binders,coating temperatures above 150° C. are advantageous. When theabovementioned binders are added, the usable coating temperatures are,depending on the binder used, from 50 to 450° C. (DE 21 06 796). Thebinders applied burn off within a short time after introduction of thecatalyst into the reactor and start-up of the reactor. The addition ofbinder has the additional advantage that the active composition adhereswell to the support so that transport and charging of the catalyst aremade easier.

Gas-phase oxidations over the abovementioned coated catalysts do nottake place only on the outer surface of the shell. To achieve thecatalyst activity and selectivity required for complete conversion ofthe high loadings of the reaction gas with starting material employed inindustrial processes, it is necessary for the total active compositionshell of the catalyst to be utilized efficiently and thus for thereaction centers located in this shell to be readily accessible to thereaction gas. Since the oxidation of aromatic compounds to givecarboxylic acids and/or carboxylic anhydrides proceeds via manyintermediates and the desired product can be further oxidized over thecatalyst to form carbon dioxide and water, optimum matching of theresidence time of the reaction gas in the active composition bygenerating a suitable active composition structure (for example itsporosity and pore radius distribution) in the catalyst shell isnecessary to achieve a high conversion of starting material while at thesame time suppressing the oxidative degradation of the desired product.

Furthermore, it has to be taken into account that the gas composition atthe outer surface of the active composition shell does not necessarilycorrespond to the gas composition at points inside the activecomposition. Rather, it is to be expected that the concentration ofprimary oxidation products is higher and the starting materialconcentration is correspondingly lower than at the outer catalystsurface. This different gas composition should be taken into account bymeans of a targeted variation of the composition of the active shellwithin this shell in order to achieve optimum catalyst activity andselectivity. Thus, DE 22 12 964 has already described a method ofsequentially spraying slurries of differing compositions onto a supportand the use of the catalysts obtained in this way for preparing phthalicanhydride.

However, the multilayer coated catalysts obtained in this way do not yetgive satisfactory results and have the disadvantage that onlyunsatisfactory yields of phthalic anhydride are achieved when they areused for the oxidation of o-xylene.

It is an object of the present invention to propose multilayer coatedcatalysts which allow a further increase in the selectivity of theoxidation of aromatic hydrocarbons to form carboxylic acids.

We have found that this object is achieved by a coated catalyst for thecatalytic gas phase oxidation of aromatic hydrocarbons, comprising, onan inert nonporous support, a catalytically active compositioncomprising, in each case based on the total amount of catalyticallyactive composition, from 1 to 40% by weight of vanadium oxide,calculated as V₂O₅, from 60 to 99% by weight of titanium dioxide,calculated as TiO₂, up to 1% by weight of a cesium compound, calculatedas Cs, up to 1% by weight of a phosphorus compound, calculated as P, andup to a total of 10% by weight of antimony oxide, calculated as Sb₂O₃,wherein the catalytically active composition is applied in two or morelayers, where the inner layer or inner layers have an antimony oxidecontent of from 1 to 15% by weight and the outer layer has, in contrast,an antimony oxide content which is from 50 to 100% lower and the amountof catalytically active composition of the inner layer or the innerlayers is from 10 to 90% by weight of the total amount of catalyticallyactive composition.

We have also found a production process for these catalysts and aprocess using these catalysts for preparing carboxylic acids and/oranhydrides and especially phthalic anhydride.

The thickness of the inner layer or the sum of the thicknesses innerlayers is generally from 0.02 to 0.2 mm, preferably from 0.05 to 0.1 mm,and that of the outer layer is generally from 0.02 to 0.2 mm, preferablyfrom 0.05 to 0.1 mm.

The novel catalysts preferably comprise two concentric layers ofcatalytically active composition, where the inner layer preferablycomprises from 2 to 10, in particular from 5 to 10% by weight, ofvanadium oxide and preferably from 2 to 7, in particular from 2.5 to 5%by weight, of antimony oxide and the outer layer preferably comprisesfrom 1 to 5, in particular from 2 to 4% by weight, of vanadium oxide andpreferably from 0 to 2, in particular from 0 to 1% by weight, ofantimony oxide.

In addition, the coated catalysts comprise further constituents whichare known per se for the oxidation of aromatic hydrocarbons tocarboxylic acids, for example titanium dioxide in the anatase formhaving a BET surface area of from 5 to 50 m²/g, preferably from 13 to 28m²/g.

The nonporous inert support comprises, for example, quartz (SiO₂),porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile,alumina (Al₂O₃), aluminum silicate, magnesium silicate (steatite),zirconium silicate or cerium silicate or a mixture of these supportmaterials. Preference is given to using steatite in the form of sphereshaving a diameter of from 3 to 6 mm or of rings having an externaldiameter of from 5 to 9 mm and a length of from 4 to 7 mm.

Apart from the optional additives cesium and phosphorus which havealready been mentioned above, it is in principle possible for thecatalytically active composition to further comprise small amounts ofmany other oxidic compounds which, as promoters, influence the activityand selectivity of the catalyst, for example by lowering or increasingits activity. Examples of such promoters are the alkali metal oxides, inparticular lithium, potassium and rubidium oxides as well as theabovementioned cesium oxide, thallium(I) oxide, aluminum oxide,zirconium oxide, iron oxide, nickel oxide, cobalt oxide, manganeseoxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenumoxide, tungsten oxide, iridium oxide, tantalum oxide, niobium oxide,arsenic oxide, antimony oxide and cerium oxide. However, from among thisgroup, cesium is generally used as promoter. Further preferred additivesfrom among the abovementioned promoters are the oxides of niobium,tungsten and lead in amounts of from 0.01 to 0.50% by weight, based onthe catalytically active composition. Suitable additives for increasingthe activity but reducing the selectivity are, especially, oxidicphosphorus compounds, in particular phosphorus pentoxide.

In general, the inner layer of the catalyst is phosphorus-containing andthe outer layer is low in phosphorus or phosphorus-free.

The application of the individual layers of the coated catalyst on theinert nonporous support can be carried out using any methods known perse, for example by

(a) spraying-on of solutions or suspensions in a coating drum,

(b) coating with a solution or suspension in a fluidized bed or

(c) powder coating of the supports.

With regard to (a)

The sequential spraying-on is generally carried out as described in DE22 12 94 and EP 21325, with the proviso that chromatographic effects,i.e. the migration of individual constituents into the other layer,should be avoided as far as possible. If the active components to beapplied are not at least partly present as insoluble metal compounds, itmay be advantageous for this purpose to subject the powders to beapplied to a thermal pretreatment or to make them virtually insoluble inanother way, e.g. by means of additives.

With regard to (b)

The coating in a fluidized bed can be carried out as described in DE 1280 756.

With regard to c)

The method of powder coating, which is known from WO-A 98/37967 and EP-A714 700, can also be employed for sequential coating in a plurality oflayers. For this purpose, powders are first prepared from the solutionand/or suspension of the catalytically active metal oxides, with orwithout addition of auxiliaries, and these powders are applied insuccession, with or without heat treatment in between, in the form of ashell to the support.

To remove volatile constituents, the catalyst is generally, at leastafterwards, subjected to a heat treatment.

The novel catalysts are generally suitable for the gas-phase oxidationof aromatic C₆-C₁₀-hydrocarbons such as benzene, the xylenes, toluene,naphthalene or durene (1,2,4,5-tetramethylbenzene) to give carboxylicacids and/or carboxylic anhydrides, e.g. maleic anhydride, phthalicanhydride, benzoic acid and/or pyromellitic dianhydride.

In particular, the novel coated catalysts make possible a significantincrease in the selectivity and yield in the preparation of phthalicanhydride.

For this purpose, the catalysts produced according to the presentinvention are introduced into reaction tubes which are thermostattedfrom the outside to the reaction temperature, for example by means ofsalt melts, and the reaction gas is passed over this catalyst bed attemperatures of generally from 300 to 450, preferably from 320 to 420and particularly preferably from 340 to 400° C., and a gauge pressure ofgenerally from 0.1 to 2.5, preferably from 0.3 to 1.5 bar, and at aspace velocity of generally from 750 to 5000 h⁻¹.

The reaction gas fed to the catalyst is generally produced by mixing agas comprising molecular oxygen and, if appropriate, suitable reactionmoderators or diluents, e.g. steam, carbon dioxide and/or nitrogen, withthe aromatic hydrocarbon to be oxidized. The gas comprising themolecular oxygen generally comprises from 1 to 100, preferably from 2 to50 and particularly preferably from 10 to 30 mol %, of oxygen, from 0 to30, preferably from 0 to 10 mol %, of water vapor, from 0 to 50,preferably from 0 to 1 mol %, of carbon dioxide and nitrogen as balance.To produce the reaction gas, the gas comprising molecular oxygen isgenerally mixed with from 30 to 150 g of the aromatic hydrocarbon to beoxidized per standard m³ of gas.

In carrying out the gas-phase oxidation, it is advantageous tothermostat two or more zones, preferably two zones, of the catalyst bedlocated in the reaction tube to different reaction temperatures, forwhich purpose it is possible to use, for example, reactors havingseparate salt baths, as described in DE-A 22 01 528 or DE-A 28 30 765.If the reaction is carried out in two reaction zones, as described inDE-A 40 13 051, the reaction zone located toward the end at which thereaction gas enters, which zone generally makes up from 30 to 80 mol %of the total catalyst volume, is generally thermostatted to a reactiontemperature which is from 1 to 20 higher, preferably from 1 to 10 and inparticular from 2 to 8° C. higher, than that of the reaction zonelocated toward the gas outlet end. Alternatively, the gas-phaseoxidation can also be carried out at one reaction temperature withoutdivision into temperature zones. Regardless of the temperaturestructure, it has been found to be particularly advantageous to usecatalysts which differ in their catalytic activity and/or the chemicalcomposition of their active shell in the abovementioned reaction zonesof the catalyst bed. When using two reaction zones, the catalyst used inthe first reaction zone, i.e. that located toward the end at which thereaction gas enters, is preferably one which has a somewhat lowercatalytic activity than the catalyst located in the second reactionzone, i.e. the reaction zone located toward the gas inlet end. Ingeneral, the reaction is controlled by the temperature profile so thatthe major part of the aromatic hydrocarbon present in the reaction gasis reacted at maximum yield in the first zone.

If the preparation of PA is carried out using the catalysts of thepresent invention and a plurality of reaction zones in which differentcatalysts are present, the novel coated catalysts can be used in allreaction zones. However, considerable advantages over conventionalprocesses can generally be achieved even if a coated catalyst accordingto the present invention is used only in one of the reaction zones ofthe catalyst bed, for example the first reaction zone, and coatedcatalysts produced in a conventional way are employed in the otherreaction zones, for example the second or last reaction zone.

EXAMPLES

The anatase employed contains: 0.18% by weight of S, 0.08% by weight ofP, 0.24% by weight of Nb, 0.01% by weight of Na, 0.01% by weight of K,0.004% by weight of Zr, 0.004% by weight of Pb.

Example 1 Production of Coated Catalyst Ia—Comparison

A suspension comprising 250.0 g of anatase having a BET surface area of20 m²/g, 13.6 g of vanadyl oxalate (=7.98 g of V₂O₅), 1.37 g of cesiumsulfate (=1.01 g of Cs), 940 g of water and 122 g of formamide was driedin a spray dryer at a gas inlet temperature of 280° C. and a gas outlettemperature of the drying gas (air) of 120° C. to produce 270 g ofpowder having a particle size of from 3 to 60 μm for 90% by weight ofthe powder. After calcination of the powder (1 hour at 400° C.), 90 g ofthe calcined powder were mixed with 10 g of melamine. 700 g of steatite(magnesium silicate) rings having an external diameter of 8 mm, a lengthof 6 mm and a wall thickness of 1.5 mm were coated with 93 g of themelamine-containing powder with addition of 56 g of a mixture of 30% byweight of water and 70% by weight of glycerol in a coating drum at 20°C. for 20 minutes. The catalyst support which had been coated in thisway was subsequently dried at 25° C. After heat treatment at 400° C. for½ hour, the weight of the catalytically active composition applied inthis way was 10.7% by weight, based on the total weight of the finishedcatalyst. The catalytically active composition which had been applied,i.e. the catalyst shell, comprised 0.40% by weight of cesium (calculatedas Cs), 3.0% by weight of vanadium (calculated as V₂O₅) and 96.6% byweight of titanium dioxide.

Example 2 Production of Coated Catalyst Ib—Comparison

The procedure of Example 1 was repeated, but using a suspensioncomprising 400.0 g of anatase having a BET surface area of 21 m²/g, 57.6g of vanadyl oxalate (=33.8 g of V₂O₅), 2.75 g of cesium sulfate (=2.02g of Cs), 14.4 g of antimony trioxide, 2.5 g of ammonium dihydrogenphosphate (=0.67 g of P), 1500 g of water and 196 g of formamide. Thecatalytically active composition applied comprised 0.15% by weight ofphosphorus (calculated as P), 7.5% by weight of vanadium (calculated asV₂O₅), 3.2% by weight of antimony (calculated as Sb₂O₃), 0.45% by weightof cesium (calculated as Cs) and 89.05% by weight of titanium dioxide.

Example 3 Production of Coated Catalyst Ic—Comparison

The procedure of Examples 1 and 2 was repeated, except that 46 g of thepowder described in Example 1 were applied to the support first and 47 gof the powder described in Example 2 were applied subsequently.

Example 4 Production of Coated Catalyst Id—Comparison

700 g of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 400.0g of anatase having a BET surface area of 20 m²/g, 57.6 g of vanadyloxalate (=33.8 g of V₂O₅), 14.4 g of antimony trioxide, 2.5 g ofammonium dihydrogen phosphate (=0.67 g of P), 2.44 g of cesium sulfate(=1.79 g of Cs), 618 g of water and 128 g of formamide until the weightof the layer applied was 10.5% of the total weight of the finishedcatalyst. The catalytically active composition applied in this way, i.e.the catalyst shell, comprised, on average, 0.15% by weight of phosphorus(calculated as P), 7.5% by weight of vanadium (calculated as V₂O₅), 3.2%by weight of antimony (calculated as Sb₂O₃), 0.4% by weight of cesium(calculated as Cs) and 89.05% by weight of titanium dioxide.

Example 5 Production of Catalyst IIa—According to the Present Invention

The procedure of Example 3 was repeated, except that 46 g of the powderdescribed in Example 2 were applied to the support first and 47 g of thepowder described in Example 1 were applied subsequently.

Example 6 Production of Catalyst IIb—According to the Present Invention

The procedure of Example 5 was repeated, but with the modification thatthe powder prepared as described in Example 2 comprised 61.5 g insteadof 57.6 g of vanadyl oxalate.

Example 7 Production of Catalyst IIc—According to the Present Invention

The procedure of Example 5 was repeated, but with the modification thatthe powder prepared as described in Example 2 comprised 20.02 g insteadof 14.4 g of antimony trioxide.

Example 8 Production of Catalyst IId—According to the Present Invention

The procedure of Example 5 was repeated, but with the modification thatthe powder prepared as described in Example 2 comprised 9.0 g instead of14.4 g of antimony oxide.

Example 9 Production of Catalyst IIe—According to the Present Invention

700 g of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 400.0g of anatase having a BET surface area of 20 m²/g, 57.6 g of vanadyloxalate (=33.8 g of V₂O₅), 14.4 g of antimony trioxide, 2.5 g ofammonium dihydrogen phosphate (=0.67 g of P), 2.44 g of cesium sulfate(=1.79 g of Cs), 618 g of water and 128 g of formamide until the weightof the layer applied was 5.3% of the total weight of the finishedcatalyst. These precoated rings were subsequently sprayed with asuspension comprising 400.0 g of anatase having a BET surface area of 20m²/g, 30.7 g of vanadyl oxalate, 2.45 g of cesium sulfate, 618 g ofwater and 128 g of formamide until the weight of the layer applied was10.6% of the total weight of the finished catalyst.

Example 10 Production of Catalyst III—Not According to the PresentInvention

700 g of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 400.0g of anatase having a BET surface area of 20 m²/g, 57.6 g of vanadyloxalate, 14.4 g of antimony trioxide, 2.5 g of ammonium dihydrogenphosphate, 0.61 g of cesium sulfate, 618 g of water and 128 g offormamide until the weight of the layer applied was 10.5% of the totalweight of the finished catalyst. The catalytically active compositionapplied in this way, i.e. the catalyst shell, comprised 0.15% by weightof phosphorus (calculated as P), 7.5% by weight of vanadium (calculatedas V₂O₅), 3.2% by weight of antimony (calculated as Sb₂O₃), 0.1% byweight of cesium (calculated as Cs) and 89.05% by weight of titaniumdioxide.

Example 11 Preparation of PA—According to the Present Invention andComparison

An iron tube having a length of 3.85 m and an internal diameter of 25.2mm was charged with 1.30 m of catalyst III and subsequently with 1.60 mof one of the catalysts Ia-Id or IIa-IIe. To regulate the temperature,the iron tube was surrounded by a salt melt. 4.0 standard m³/h of airhaving loadings of 98.5% by weight purity o-xylene of from 0 to about 85g/standard m³ of air were passed through the tube from the top downward.At a loading of 75-85 g, the results summarized in the following tablewere obtained (yield=the amount of PA obtained in % by weight, based on100%-pure o-xylene).

Composition of the (individual) layers* Salt bath Phthalide layer V₂O₅Sb₂O₃ Cs P TiO₂ tempe- content of the Cata- (O = outer; (% by (% by (%by (% by (% by rature PA yield crude PA lyst I = Inner) weight) weight)weight) weight) weight) (° C.) (% by weight) (% by weight) Ia O = I 3.00.40 96.6 350-352 113.2 0.17 comp. Ib O = I 7.5 3.2 0.45 0.15 88.7357-359 113.5 0.30 comp. Ic O 7.5 3.2 0.45 0.15 88.7 357-360 112.9 0.16comp. I 3.0 0.40 96.6 Id O = I 7.5** 3.2 0.40 0.15 88.7 360-361 112.50.15 comp. IIa O 3.0 0.40 96.6 353-354 114.2 0.13 I 7.5 3.2 0.45 0.1588.7 IIb O 3.0 0.40 96.6 354-355 114.6 0.18 I 8.0 3.2 0.45 0.15 88.7 IIcO 3.0 0.40 96.6 355-356 114.0 0.20 I 8.0 4.5 0.45 0.15 88.7 IId O 3.00.40 96.6 354-355 114.1 0.15 I 8.0 2.0 0.45 0.15 88.7 IIe O 4.0** 0.4095.6 352-353 114.6 0.19 I 7.5** 3.2 0.40 0.15 88.7 *Due to abrasioneffects during the coating procedure, there is no abrupt change inchemical composition between the two layers, but rather there is agradual transition from one chemical composition to the other; in theextreme case, traces of the active powder of the inner layer are alsopresent in the outer layer. Further constituents of the activecomposition which can be detected by analysis and originate fromimpurities in the anatase used are not shown. **As a result of theproduction method, chromatographic effects occur in these examples andlead, depending on the method of drying, to different vanadiumconcentration profiles; it is therefore only possible to give an averagevanadium concentration.

We claim:
 1. A coated catalyst for the catalytic gas-phase oxidation ofaromatic hydrocarbons, comprising, on an inert nonporous support, acatalytically active composition comprising, in each case based on thetotal amount of catalytically active composition, from 1 to 40% byweight of vanadium oxide, calculated as V₂O₅, from 60 to 99% by weightof titanium dioxide, calculated as TiO₂, up to 1% by weight of a cesiumcompound, calculated as Cs, up to 1% by weight of a phosphorus compound,calculated as P, and a total of from more than 0 up to 10% by weight ofantimony oxide, calculated as Sb₂O₃, wherein the catalytically activecomposition is applied in two or more layers, where the inner layer orinner layers have an antimony oxide content of from 1 to 15% by weightand the outer layer has, in contrast, an antimony oxide content which isfrom 50 to 100% lower, the amount of catalytically active composition ofthe inner layer or the inner layers is from 10 to 90% by weight of thetotal amount of catalytically active composition and the amounts of theconstituents of the catalytically active composition are to be selectedfrom the stated ranges such that they sum to 100% by weight.
 2. A coatedcatalyst as claimed in claim 1, wherein the catalytically activecomposition of the inner layer or the sum of the inner layers is from 30to 70% by weight of the total amount of catalytically active compositionof the catalyst.
 3. A coated catalyst as claimed in claim 1, wherein thethickness of the inner layer or the sum of the thicknesses of the innerlayers is from 0.02 to 0.2 mm and the thickness of the outer layer isfrom 0.02 to 0.2 mm.
 4. A coated catalyst as claimed in claim 1, whereinthe catalyst has two concentric layers of catalytically activecomposition, where the inner layer contains from 2 to 7% by weight ofantimony oxide and the outer layer contains from 0 to 2% by weight ofantimony oxide.
 5. A coated catalyst as claimed in claim 1, wherein thecatalyst has two concentric layers of catalytically active composition,where the inner layer contains from 5 to 10% by weight of vanadium oxideand the outer layer contains from 1 to 5% by weight of vanadium oxide.6. A coated catalyst as claimed in claim 1, wherein the material of theinert nonporous support is steatite.
 7. A coated catalyst as claimed inclaim 1, wherein the titanium oxide therein is present as titaniumdioxide in the anatase form and has a BET surface area of from 13 to 28m²/g.
 8. A coated catalyst as claimed in claim 1, wherein two concentriclayers of catalytically active composition are applied in the form of ashell to an inert nonporous steatite support, where, apart from titaniumdioxide in the anatase form having a BET surface area of from 13 to 28m²/g, the inner layer comprises from 5 to 10% by weight of vanadiumoxide, calculated as V₂O₅, and from 2 to 7% by weight of antimony oxide,calculated as Sb₂O₃, and the outer layer comprises from 1 to 5% byweight of vanadium oxide, calculated as V₂O₅, and from 0 to 2% by weightof antimony oxide, calculated as Sb₂O₃.
 9. A process for producingcoated catalysts as claimed in claim 1, which comprises applying two ormore than two layers of the catalytically active composition insuccession to an inert nonporous support by spraying.
 10. A process forproducing coated catalysts as claimed in claim 1, which comprisesapplying two or more than two layers of the catalytically activecomposition in succession to an inert nonporous support by coating withthe binder-containing catalytically active composition in powder form.11. A process for preparing carboxylic acids and/or carboxylicanhydrides by the partial oxidation of aromatic hydrocarbons whichcomprises contacting an aromatic hydrocarbon with gases containingmolecular oxygen in the presence of a catalyst as defined in claim 1.12. A process for preparing phthalic anhydride by the partial oxidationof o-xylene and/or naphthalene which comprises contacting o-xyleneand/or naphthalene with gases containing molecular oxygen in thepresence of a catalyst as defined in claim 1.